1. Field of the Invention
The invention relates to the field of audio power amplifiers and, more particularly, to class-B current source audio power amplifiers.
2. Discussion of the Prior Art
With the advent of germanium and silicon power transistors, audio amplifier design rapidly improved due to the elimination of the prior expensive and cumbersome output transformers. However, inherent difficulties of solid state power amplifiers presented themselves and such difficulties included poor linearity of the power transistors and their associated temperature instability.
Modern and conventional power audio amplifiers are primarily a voltage source type (i.e. constant voltage) and the majority of this type have their power transistors connected as emitter-followers for best linearity. With this kind of design, the driving stage must, besides furnishing a high fidelity driving voltage, which is the same magnitude as the output voltage, carry the burden of providing the necessary bias for the output power transistors. Thus, over biasing of the power transistors will cause excessive heating and under biasing of the power transistors will cause distortion. Various techniques have been used to solve these problems. These solutions are usually massive, complicated, expensive and inefficient. For example, purposefully over biasing the power transistor improves fidelity and, large heat sinks are used around the power transistors to maintain temperature stability thereby minimizing any load variations. In this regard, the power transistors are purposely operated at a higher temperature level so that the power transistors are placed in massive heat sinks thereby minimizing changes caused by temperature due to ambient temperature and load variations. In addition, complicated arrangements have been generated for the thermal protection of the power transistors.
Modern amplifier designs have sought to reduce any type of distortion such as intermodulation, harmonic, cross-over and phase distortions. However, a more subtle variety of distortion has now presented itself termed transient intermodulation distortion.
In ordinary class-B amplifier design enough overlap presents itself at the cross-over region that there is no cross-over distortion under steady state tests. However, under transient conditions, the cross-over region breaks down to display a kind of distortion similar to cross-over distortion now termed transient intermodulation distortion. Prior art remedies are (1) to operate the amplifier as class-AB (excessive overlap) where the overlap at the cross-over region is excessive, (2) to operate as the amplifier as class-A with 100% overlap, and (3) to distribute feedback at different stages of amplification. Amplifiers using remedies (1) and (2) above exhibit excessive heat loss.
In Jan Lohstroh and Matti Otala's article entitled "An Audio Power Amplifier for Ultimate Quality Requirements", published in the IBEE Transactions on Audio and Electro-acoustics, Volume AU-21, No. 6, December 1973, the authors discuss transient intermodulation distortion and propose a complimentary symmetrical audio amplifier minimizing all distortion including transient intermodulation distortion. This article provides a good discussion of the background of the prior art with relation to audio amplifier design and distortion. The amplifiers presented and discussed in this article, however, relate to voltage type power amplifiers and not to current source amplifiers.
One of the most common forms of audio amplifier design is the class-B. The final stage of amplification in this class of amplification is split into two halves so that at any given moment only one half is producing sound. This results in an efficient method of amplification since no one transistor or output tube need do all the work by itself. However, the two halves, in a class-B amplifier, must be rigidly controlled by other circuits and must be very carefully matched in characteristics or considerable distortion can result.
In the theoretical sense, an ideal class-B amplifier amplifies only one half of a cycle at a time and as illustrated in FIG. 1, the quiescent dc current, I.sub.o, is zero. However, the presence of loading current, I.sub.l, through each amplifier half causes heating.
In a practical sense, cross-over distortion is always present and a residual current, I.sub.o, is allowed to flow. The practical result is shown in FIG. 2 where the positive waveform overlaps with the negative waveform to eliminate cross-over distortion. As can be seen, the operation cycle of each amplifier half is actually greater than 1/2 cycle. And, in this situation, the dc component is now: EQU I.sub.t =I.sub.L +I.sub.o
It is evident that the output power transistor for each half operates at a higher temperature due to the combined values of I.sub.L and I.sub.o. When the load is removed, the amplifier returns to its quiescent condition and two possibilities present themselves. First, and most common, the output current I.sub.t will return to its original value of I.sub.o and, second, I.sub.o will progressively increase and cause output power transistor failure. In practical class-B amplifiers, the two halves are carefully constructed with components having identical characteristics.
U.S. Pat. No. 3,542,952, entitled "Low Distortion Signal Reproduction Apparatus", issued on Nov. 24, 1970, to this inventor sets forth a novel current source amplifier design. The present invention incorporates the teachings of this earlier patent issued to the inventor and produces a true class-B current source amplifier with substantially reduced intermodulation, harmonic, cross-over, phase and transient intermodulation distortions. It is important to understand that these terms have been generated to classify distortion appearing in voltage power amplifiers and the use of these terms to describe distortion in current source amplifiers may be inappropriate. In any event, the present invention minimizes any and all distortion.
The cross-over region overlap determines the heating of the power transistor. In class-AB operation, the heating is very high (as explained before) and excessive overlap is used to reduce transient intermodulation distortion. In ordinary amplifier (voltage type), the coupling between amplifier and loudspeaker load is loose, under transient condition there is a tendency to break loose causing transient distortion. In the current source amplifier set forth below, the coupling between two is stiff and, therefore, the current source amplifier can be operated at minimum overlap for low distortion with little heating.
The present invention, contrary to the entire thrust of audio design which is concentrating on voltage feedback and voltage power amplifier arrangements, utilizes a current source amplifier (constant current type) with a single feed-back loop. The amplifier is of the class-B type and has an extremely high open-loop gain, extremely low distortions, excellent transient response, and overall temperature stability. Under the teachings of this invention, a true class-B audio power amplifier is realized where the cross-over overlap is adequate for low distortion but is minimized for low heating of power transistors. The amplifier is designed with current coupled transistor stages instead of the conventional emitter-follower type with an extremely high open-loop gain. This is not possible to achieve with the conventional emitter-follower amplifier design principles.
A novel biasing scheme and principle is introduced in the present invention for current source audio power amplifiers where one amplifier half is fixedly biased while the other amplifier half is slave biased to the first half in order to maintain the dc output voltage level at zero. Thus, in reference to FIG. 2, in the present invention the heating of the power transistors due to load or due to the environment automatically reduces I.sub.o to check the progressive increase of I.sub.o due to leakage. Since, in the present invention, the negative amplifier half is slave-biased to the positive amplifier half, only I.sub.o in the positive half is adjusted which causes a corresponding adjustment of -I.sub.o in the negative half. More importantly, the two halves in the present invention are automatically balanced so that there is no resultant dc load current at the output.
This is not possible with the conventional class-B amplifiers where the biases of the two amplifier halves cannot be separated. The amplifier of the present invention is separately biased completely contrary to the prior art and approaches. Each amplifier half of the present invention is composed of a tandem chain of transistors direct current coupled to achieve high current gain. The first half is termed the positive half which delivers the positive going output signals and the second half is termed the negative half to deliver the negative going output signals.
In another principle of this invention, since the negative amplifier half is slave biased to the positive amplifier half, the corresponding transistors need not be matched in characteristics. This results in a considerable cost reduction and improvement over prior art approaches.
In another principle of this invention, as mentioned, the temperature of only one of the power transistors is sensed. When the temperature of the power transistor increases due to increasing load demands, the input biasing current to the amplifier is reduced. Therefore, as one power transistor gets warm or hot the temperature sensor causes the bias to both power transistors to be automatically reduced. As the temperature of the one power transistor decreases, the bias currents to both are automatically increased. Only one power transistor (on the positive half) need be sensed since the two amplifier halves operate in a slave biased arrangement.
Another principle of the present invention relates to a temperature sensor for sensing the ambient temperature of the amplifier. Like the load temperature sensor, the input bias is automatically adjusted to maintain the amplification. Therefore, the two amplifier halves of the present invention work in unison to maintain the output voltage at zero volts dc, regardless of operating levels, power transistor mismatches, and temperature variations.
The circuitries presented are simple and straight forward to be assembled and manipulative and, moreover, the amplifier is efficient, inexpensive to build, and reliable. Moreover, with a high open-loop gain, the fidelity and transient responses are extremely excellent.